In recent years, a distance measurement device for measuring a distance to a subject (object to which a distance is to be measured) by a parallax between a plurality of imaging optical systems has been used in an inter-vehicle distance measurement system, a camera auto-focus system, and a three-dimensional shape measurement system.
In this type of measurement device, an image is formed in each imaging area by a pair of imaging optical systems respectively arranged on left and right sides or one above the other, and the distance to the subject is detected from a parallax between the images by using triangulation.
On the other hand, as a method of measuring the distance to the subject from a single imaging optical system, a depth from defocus (DFD) method has been known. In the DFD method, the distance is calculated from an analysis of blurring amount of an acquired image. However, it is hard to determine whether the detection result is a pattern of the subject itself or a blurring due to the distance to the subject with a single image, and therefore, a method of estimating the distance from a plurality of images has been employed (see Patent Document No. 1 and Non-patent Document No. 1).